On filter assembly machines, filter portions, of a length equal to that of an even number of filters, normally four or six, are fed into a hopper, from which they are withdrawn by an extracting drum having a number of peripheral seats equally spaced about the extracting drum and for receiving and retaining respective filter portions by suction. Each of the filter portions is fed by the extracting drum through a cutting station where it is cut into a number of shorter filter portions defining respective double filters, i.e. twice the length of a cigarette filter, positioned coaxially inside the respective seat.
The double filters in each number are then transferred to an offsetting drum, which offsets them angularly with respect to one another to form, along its periphery, a number of rows of double filters equal to the number of double filters formed from each filter portion. The double filters in each row are equally spaced with a first spacing, which is equal for all the rows, about the axis of the offsetting drum, and each double filter in each row is offset, with respect to a corresponding double filter in an adjacent row, by a second spacing equal to a submultiple of the first spacing.
The offset double filters are then fed to a centring drum, which shuffles the rows, by shifting them laterally, into a single row in which the double filters are spaced with said second spacing. This single row is then fed in known manner to a feed line supplying cigarette portions, to form double cigarettes.
On known centring drums, the rows are normally shuffled by means of fixed external converging plates, which gradually engage the rows of double filters, and slide the double filters axially along the relative seats into alignment with one another and into a central position normally centred with respect to a reference plane crosswise to the rotation axis of the centring drum.
Though perfectly functional, known centring drums of the type described above have drawbacks when making any change in format, which normally involves changing and/or dismantling and reassembling said plates, thus resulting in relatively prolonged downtime.